The impact of climate and tectonics on sedimentary and deformational processes, Gulf of Alaska

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Abstract

Collision of the Yakutat Terrane with North America in southern Alaska has
driven growth of the Chugach-St. Elias orogen. Glaciation of the St. Elias Range has
periodically increased since the Miocene, but began dominating erosion and spurred
enhanced exhumation since the mid-Pleistocene transition at ~1 Ma. Ice associated with this glacial intensification carved cross-shelf sea valleys that connect the St. Elias Range to the deep-sea Surveyor Fan. A newly increased terrigenous sediment flux into the fan triggered the formation and growth of the Surveyor Channel. The change in
geomorphology observed throughout Fan sequences allows us to characterize the
influence that a glaciated orogen can have in shaping margin processes and the sediment
pathways from source to sink. Seismic data also reveal an isolated, large, short runout,
mass-transport deposit (MTD) buried in the Surveyor Fan. The MTD geometry, size and location on a convergent margin lend support to recent studies suggesting seismic
strengthening and infrequent sediment failure on active margins. This study provides
insight into the magnitude and scope of events required to cause submarine mega-slides
and overcome higher than normal sediment shear strength, including the influence of
climate and sea level change. Beneath the Surveyor Fan, integrated geophysical data
reveals massive intraplate shearing, and a lack of oceanic crust magnetic lineaments in
regions of Pacific Plate crust. We argue that stress from the Yakutat-North America
collision transferred outboard to the Pacific Plate is the major driver for the deformation
causing these features. This stress would have resulted in significant strain in the NE
corner of the Pacific Plate, creating pathways for sill formation in the crust and Surveyor
Fan. The collision further intensified as the thickest Yakutat portion began to subduct
during the Pleistocene, possibly providing the impetus for the creation of the Gulf of
Alaska Shear Zone, a >200 km zone of shear extending out into the Pacific Plate. This
study highlights the importance of farfield stress from complex tectonic regimes in
consideration of large-scale oceanic intraplate deformation.